The invention relates to a pivoting power transmission unit, to drive in rotation, from at least one source of power in rotation, at least one driven component, designed to rotate about an axis of rotation which itself, during rotation of the driven component, has to swivel about a pivot axis substantially perpendicular to the axis of rotation of the driven component.
The pivoting power transmission unit according to the invention can be used, in a non-limiting manner, as a power transmission unit fitted to aircraft on which the propulsion system is swivellable.
In particular, as an embodiment for which the pivoting power transmission unit according to the invention is of great relevance for the applicant, such a power transmission unit can be used to drive in rotation, from at least one aircraft power unit, of the turboshaft engine type, for example, at least one rotor of an aircraft of the convertible type, which can operate in aeroplane mode or in helicopter mode, and in which the rotor drive shaft swivels about a pivot axis so that it can move from one position, in which the rotor operates as an aircraft propeller, for the aircraft to fly in aeroplane mode, to a position in which the rotor operates as a helicopter lift rotor, for the aircraft to fly in helicopter mode.
A pivoting power transmission unit for such a convertible aircraft with tilting rotors is described in particular in FR 2 791 319 and FR 2 791 634 to which reference should be made for further details.
It is pointed out however that these two patents describe a convertible aircraft with tilting rotors of the type in which the shaft of each rotor is driven in rotation about itself by a front reduction gear unit of one respectively of two power transmissions each also comprising a rear reduction gear unit linked to the corresponding front reduction gear unit, and also to one respectively of two (turboshaft) engines, each supported by one respectively of the two fixed wings of the aircraft, and to an interconnecting shaft linking the two transmissions for the two rotors to be driven in rotation by any one of the two engines in case of failure of the other engine.
The shaft of each rotor, the corresponding power transmission and the corresponding engine are housed in one respectively of two articulated pods, each comprising a front part mounted, so as to pivot about the pivot axis, on a stationary rear part, fixed to one respectively of the two fixed wings of the aircraft, and in which are housed the corresponding engine and at least in part the rear reduction gear unit of the corresponding transmission, the front reduction gear unit of which, embodied as a helicopter main gearbox, and also the shaft of the corresponding rotor are housed in the front pivoting part of the pod, and are mounted so as to pivot with this front part of the pod relative to the rear stationary part of the pod and the corresponding fixed wing.
FR 2 791 319 describes several modes of embodiment of the transmission of power between an engine, the corresponding rotor and the corresponding end of the interconnecting shaft (see FIGS. 5 to 10), but, in all cases, the pivoting reduction gear unit of each transmission, arranged as a helicopter main gearbox, comprises an output reduction gear stage of the epicyclic type, linked to the shaft of the corresponding rotor, and an input reduction gear stage, of the bevel or spiral bevel type, in which a bevel input pinion is coaxial with a cylindrical pinion pivoting about the pivot axis, and is joined with this pivoting pinion in rotation in at least one direction of rotation, while the rear non-pivoting reduction gear unit of each transmission is arranged as an intermediate gearbox, comprising a high-speed reduction gear stage comprising bevel or spiral bevel gears, the input bevel pinion of which is linked to an output shaft of the corresponding engine, and an intermediate reduction gear stage, engaging with the high-speed reduction gear stage and with the pivoting pinion, and also linked to the interconnecting shaft by a corresponding power takeoff, the intermediate reduction gear stage and also the pivoting pinion and the power takeoff linked to the interconnecting shaft consisting of a set of cylindrical pinions arranged as a lateral gear train relative to the output shaft of the engine and to the pivoting reduction gear unit, so as to leave sufficient space free between the front pivoting reduction gear unit and the rear non-pivoting reduction gear unit to allow the pivoting movements of the first with respect to the second of the two reduction gear units mentioned above.
The architecture of each transmission is therefore complex, and comprises a large number of pairs of teeth engaging with each other, to obtain the desired overall reduction ratio, between an output shaft of the engine revolving at a speed of rotation of the order of 20,000 rpm and the shaft of the rotor revolving at a nominal speed of between about 200 and 300 rpm, and to transmit the substantial drive torque required to the rotor shaft. Moreover, some of the reduction gear stages, particularly the epicyclic stage and the at least two bevel or spiral bevel gears required in each transmission, are bulky stages, heavily loaded and therefore dimensioned accordingly, involving high production costs and skilled maintenance because of the precision axial and radial adjustments required to position them and the resulting maintenance and interchangeability difficulties when working on these gears.
The present invention is intended to remedy the above disadvantages using an advanced pivoting power transmission unit equipped with peripheral face toothed gearwheels of the type known as face gears.
These peripheral face toothed gearwheels, sometimes also known as face gears or face-toothed gears, or again side tooth gears, are gearwheels on which one of the lateral faces has on its periphery teeth such as described in particular in EP 227 152, to which reference may advantageously be made for information on the milling of such teeth and the advantages deriving from them.
The use of peripheral face toothed gearwheels of the face gear type in power transmissions in aviation has already been proposed, in particular for fitting to single-turbine or multi-turbine helicopter main gearboxes, particularly in EP 234 058, EP 971 155 and U.S. Pat. No. 6,260,429, which present the advantages resulting from the use of such peripheral face toothed gearwheels in these particular applications, in terms of reduction in size and weight, higher reduction ratios, easier gear positioning and interchangeability and therefore improved reliability and easier maintenance.
A purpose of this invention is to make use of the same advantages in connection with a pivoting power transmission unit, no example of embodiment of which is proposed in the patents quoted above for a power transmission unit using peripheral face toothed gearwheels of the face gear type.
According to one aspect of the present invention, there is provided a pivoting power transmission unit for transmitting rotational drive from at least one source of power to at least one driven component configured to revolve about an axis of rotation and configured to be swivelled about a pivot axis substantially perpendicular to said axis of rotation, the pivoting power transmission unit comprising:
a cylindrical input pinion, configured to be driven in rotation by said at least one source of power about an axis which is substantially perpendicular to said pivot axis;
first and second gear wheels with peripheral face teeth of the face gear type which are facing each other, the first and second gear wheels being coaxial with said pivot axis and in meshing engagement with said input pinion so as to contra-rotate about said pivot axis when rotated by the input pinion;
two cylindrical intermediate pinions, each of which is joined to rotate co-axially about said pivot axis with one respectively of the first and second gear wheels; and
a third gear wheel with peripheral face teeth of the face gear type which is in meshing engagement with each of said two intermediate pinions, the third gear wheel being configured to drive in rotation an output shaft coupled to said at least one driven component.
Compared with the embodiments described in FR 2 791 319, it is noted that the pivoting power transmission unit according to the invention dispenses with the need for a lateral gear train, which plays only a small part in obtaining the required overall reduction ratio, and also eliminates the epicyclic train and the bevel or spiral bevel gears of the front pivoting reduction gear unit.
In fact, the use of the two peripheral face toothed gearwheels of the face gear type mounted so as to be coaxial and contrarotating, and of the third gearwheel of the same kind allows high reduction ratios, of at least six to seven per reduction gear stage, to be obtained with a minimum number of pairs of teeth engaging with each other.
Advantageously, the transmission unit also comprises at least one cylindrical balance pinion, with straight or helical teeth, mounted as an idler gear in-rotation about an axis substantially coplanar with the axis of the input pinion, the teeth of which mesh with the teeth of said two contrarotating gearwheels, in order to balance the forces which are introduced by the input pinion into the two coaxial and contrarotating gearwheels between which the input pinion and each balance pinion are engaged.
In a simple and effective manner, the transmission unit comprises only one balance pinion, the axis of rotation of which is substantially in the prolongation of the input pinion. In this way, the input pinion and the balance pinion revolve about coaxial axes and are diametrically opposite relative to the two coaxial and contrarotating gearwheels with which these two pinions are engaged.
Also advantageously, the third gearwheel is substantially in the shape of a hollow truncated cone, and houses parts of the two coaxial and contrarotating gearwheels at the opposite end to the input pinion and also, where applicable, the balance pinion or pinions, greatly improving the overall compactness of the transmission unit.
In a practical manner, the connection in rotation between the third gearwheel and the output shaft is provided by the fact that this third gearwheel has a central axial bore with splines engaging with axial splines on the output shaft to provide the drive in rotation of said output shaft.
Thus the pivoting power transmission unit according to the invention advantageously comprises a first reduction gear stage, comprising the input pinion, the two coaxial and contrarotating gearwheels and, where applicable, said at least one balance pinion, and a second reduction gear stage comprising the two intermediate pinions, the third gearwheel, and the output shaft, substantially coaxial with the third gearwheel, the second reduction gear stage being mounted and guided in rotation in a casing mounted so as to pivot relative to the first stage about the pivot axis, the pivoting movements of this being controlled by at least one actuator.
Also advantageously, the first reduction gear stage is mounted and guided in rotation in a stationary casing, not pivoting about said pivot axis, and on which the pivoting casing can be mounted so as to swivel about said pivot axis, and swivelled by at least one actuator supported on a stationary point on a carrier structure or said stationary casing. This actuator may be a rotary or linear actuator, of hydraulic or electro-mechanical type, and preferably, when it is a linear actuator, this actuator may be of the hydraulic jack type or electro-mechanical jack type with a ball screw or rack cooperating with a gearwheel driven in rotation by an electric motor. In the case of a linear actuator, the latter may be mounted as described and shown in the French patents quoted above, in which the linear actuator is linked at one end to an arm projecting from the pivoting casing and at the other end to a stationary point on the carrier structure which in this case is a stationary point in the stationary rear part of the corresponding pod. As a variant, the stationary point may also be on the stationary casing or on components supporting this stationary casing.
The third gearwheel is advantageously guided in rotation in the pivoting casing by at least one double-row skew-angle ball bearing, and the output shaft is guided in rotation in this same pivoting casing by at least two bearings spaced apart from each other axially.